Earth-boring tools for forming wellbores in subterranean earth formations may include a plurality of cutting elements secured to a body. For example, fixed-cutter earth-boring rotary drill bits (“drag bits”) include a plurality of cutting elements that are fixedly attached to a bit body of the drill bit. Similarly, roller cone earth-boring rotary drill bits may include cones that are mounted on bearing pins extending from legs of a bit body such that each cone is capable of rotating about the bearing pin on which it is mounted. A plurality of cutting elements may be mounted to each cone of the drill bit. Other earth-boring tools utilizing cutting elements include, for example, core bits, bi-center bits, eccentric bits, hybrid bits (e.g., rolling components in combination with fixed cutting elements), reamers, and casing milling tools.
The cutting elements used in such earth-boring tools often include a volume of polycrystalline diamond (“PCD”) material on a substrate. Surfaces of the polycrystalline diamond act as cutting faces of the so-called polycrystalline diamond compact (“PDC”) cutting elements. PCD material is material that includes inter-bonded grains or crystals of diamond material. In other words, PCD material includes direct, inter-granular bonds between the grains or crystals of diamond material. The terms “grain” and “crystal” are used synonymously and interchangeably herein.
PDC cutting elements are generally formed by sintering and bonding together relatively small diamond (synthetic, natural or a combination) grains, termed “grit,” under conditions of high temperature and high pressure in the presence of a catalyst (e.g., cobalt, iron, nickel, or alloys and mixtures thereof) to form a layer (e.g., a “compact” or “table”) of PCD material. These processes are often referred to as high temperature/high pressure (or “HTHP”) processes. The supporting substrate may comprise a cermet material (i.e., a ceramic-metal composite material) such as, for example, cobalt-cemented tungsten carbide. In some instances, the PCD material may be formed on the cutting element, for example, during the HTHP process. In such instances, catalyst material (e.g., cobalt) in the supporting substrate may be “swept” into the diamond grains during sintering and serve as a catalyst material for forming the diamond table from the diamond grains. Powdered catalyst material may also be mixed with the diamond grains prior to sintering the grains together in an HTHP process. In other methods, the diamond table may be formed separately from the supporting substrate and subsequently attached thereto.
Upon formation of the diamond table using an HTHP process, catalyst material may remain in interstitial spaces between the inter-bonded grains of the PDC. The presence of the catalyst material in the PDC may contribute to thermal damage in the PDC when the PDC cutting element is heated during use due to friction at the contact point between the cutting element and the formation. Accordingly, the catalyst material (e.g., cobalt) may be leached out of the interstitial spaces using, for example, an acid or combination of acids (e.g., aqua regia). Substantially all of the catalyst material may be removed from the PDC, or catalyst material may be removed from only a portion thereof, for example, from a cutting face of the PDC, from a side of the PDC, or both, to a desired depth. Leaching rates and uniformity may at least partially depend on the permeability of the PDC to a leaching agent. The permeability of the PDC may be influenced by the porosity and mean free path of the PDC, which are in turn influenced by average grain size and grain distribution within the PDC. When a multi-layered or multi-regioned PDC is leached, coarser layers or regions exposed to the leaching agent may exhibit accelerated leach rates as compared to finer layers or regions. Unfortunately, such accelerated leaching can result in non-uniform leach depths within the PDC, and can also lead to defective cutting elements due to undesired removal of catalyst material from a supporting substrate attached to the PDC.